A great deal of effort has been expended in attempting to find a speaker enclosure which will efficiently reproduce sounds at the low frequency end portion of the audio spectrum, which will accurately reproduce sounds without booms and other resonant effects and which is reasonable in size and not unduly expensive to manufacture. Simple enclosures in the form of boxes having closed or open backs have serious limitations with respect to response characteristics and with respect to resonance effects and distortions. Resonance effects can be attenuated through the use of acoustically absorbent materials within a cabinet but with a reduction in efficiency and other disadvantages. Folded horn type enclosures have been used with considerable success but have been quite expensive to manufacture and have not been altogether free of resonance effects even after expending great care in design and experimentation in connection therewith. Enclosures utilizing passive radiators and enclosures which are totally enclosed, i.e., accoustic suspension enclosures, have been used with considerable success, but usually at the expense of increased cost and/or reduced efficiency.
The base-reflex ported type of enclosure has been very popular because of its relative simplicity and low cost combined with very good performance. In the bass-reflex type of ported enclosure, a port opening is provided in a wall portion of an enclosure, typically in a portion of the front wall which carries the speaker behind an opening therein, spaced from the port opening. The mass of the air in the port opening cooperates with the compliance of the air volume within the enclosure to produce a phase-shift which may approach 180 degrees. Since the sound emanated from the back of the speaker cone is 180 degrees out of phase with that emanated from the front of the speaker cone, the sound emanated from the port opening may approach an in-phase relationship to the sound emanated from the front of the speaker cone.
To obtain satisfactory results with the bass-reflex ported type of enclosure, the size of the port opening should have the proper relationship to other parameters including the size of the enclosure and the size of the speaker. In general, the size of the port opening may approach that of the speaker opening for larger enclosures and is reduced to a fraction of the size of the speaker opening for smaller enclosures.
Although being generally satisfactory, prior bass-reflex ported types of enclosures have not produced the performance which can be obtained with well designed, and expensive, folded horn type enclosures acoustic suspension enclosures, or enclosures utilizing passive radiators. One problem is that undesired effects such as boominess have been produced and have been found objectional by many listeners. Such effects become more pronounced as the size of the enclosure is decreased. Some degree of improvement has been obtained through the use of ducted port enclosures in which a duct is mounted within an enclosure behind a port, the duct typically being a cylindrical cardboard tube. In such a ducted port enclosure, the size of the enclosure may be reduced but it is still possible to have a significant degree of boominess even with the addition of sound absorbing materials within the enclosure.
Other proposals have been made relating to combination of features of different types of enclosures. Thus, the front of speakers have been coupled to the throats of horns while the backs thereof have been coupled to a port in a bass-reflex enclosure. It has also been proposed to mount a horn on the outside of a bass-reflex enclosure with its mouth coupled to a port. Another arrangement uses both damping material and a port, the damping material being within the port which reduces resonance effects but which also greatly reduces efficiency and output of low frequencies.
Many other proposals have been made and some of them have been used with varying degrees of success. It would be impossible to cite all prior references of interest but they include "How to Build Speaker Enclosures" by Alexis Badmaieff and Don Davis, 1966, published by Howard W. Sams & Co. Inc.; "Elements of Acoustical Engineering" by Harry F. Olson, published by D. Van Nostrand Company, Inc. 1940; a special "Speaker Issue" of "High Fidelity" Magazine, published by ABC Leisure Magazine, Inc. 1975; Carlsson U.S. Pat. No. 3,037,081, May 29, 1962; Hedberg U.S. Pat. No. 3,547,221, Dec. 15, 1970; Hopkins U.S. Pat. No. 3,684,051, Aug. 15, 1972; Guss U.S. Pat. No. 3,688,864, Sept. 5, 1972; Klayman, et al. U.S. Pat. No. 3,892,288, July 1, 1975; and Carlsson U.S. Pat. No. 4,006,311, Feb. 1, 1977.